Electrostatic printing apparatus and method employing optical charge deposition

ABSTRACT

An electrostatic printer employing a source of coherent electromagnetic radiation in conjunction with electro-optical equipment for spatially modulating and focusing the radiation to a printing medium disposed between a first electrode transparent to the radiation and a second electrode. A focal point of radiation is established adjacent the printing medium, and the energy present there interacts with gas molecules to create ions, which are attracted to the medium by an electric field established between the electrodes. Manipulation of this point source of charge by electro-optical methods in response to control signals facilitates formation of a charge image on the medium, which may then be developed into a final print.

Unite States Patent [191 Cola [ June 10, 1975 [75] Inventor: Rudolph A. Cola, Malvern, Pa.

[73] Assignee: Burroughs Corporation, Detroit,

Mich.

[22] Filed: June 7', 1973 [21] Appl. No.: 367,736

[52] US. Cl. 346/74 J [51] Int. Cl. G0ld 15/06 [58] Field of Search 346/74 ES, 1, 741 B; 250/423, 424, 199

[56] References Cited UNITED STATES PATENTS 3,358.081 12/1967 Young 178/66 R 3,478,204 11/1969 Brubaker 250/423 3,611,405 10/1971 Chang 346/1 3,611,418 10/1971 Uno et a1. 346/74 ES 3,673,598 6/1972 Simm 346/74 ES Primary Examiner-Bernard Konick Assistant ExaminerJay P. Lucas Attorney, Agent, or Firm-Franklin D. Ubell; William B. Penn; Kevin R. Peterson [5 7 ABSTRACT An electrostatic printer employing a source of coherent electromagnetic radiation in conjunction with electro-optical equipment for spatially modulating and focusing the radiation to a printing medium disposed between a first electrode transparent to the radiation and a second electrode. A focal point of radiation is established adjacent the printing medium, and the energy present there interacts with gas molecules to create ions, which are attracted to the medium by an electric field established between the electrodes. Manipulation of this point source of charge by electrooptical methods in response to control signals facilitates formation of a charge image on the medium, which may then be developed into a final print.

2 Claims, 4 Drawing Figures PATENTEDJUM 0 1915 SHEET 1 F i g. 1

Control 14 Signal Source Print Laser Modulator DefIeCEiOD Station ELECTROSTATIC PRINTING APPARATUS AND METHOD EMPLOYING OPTICAL CHARGE DEPOSITION BACKGROUND OF THE INVENTION This invention relates generally to electrostatic printing devices, and specifically to the production of an image in such a device by the deposition of charged particles on a printing medium in the presence of an electric field. The invention provides electro-optical apparatus for generating a highly manipulable, localized and transient point charge source applicable in a charge image deposition.

In the prior-art, the basic approach to electrostatic charge deposition printing has employed charged particle discharge sources fixed opposite a printing medium and stimulated to emit charge subject to the direction of control signals. Electric fields have served to direct the emitted charge to a printing medium to form a charge image, which may be developed into a final print.

In general, the operation of such prior art devices has suffered impairment arising from complex control apparatus, atmospheric exposure and geometrical constraints. For instance, one prior approach, as exemplifie'd by US. Pat. No. 2,955,894, issued to H. Epstein on Oct. 1 1, 1960, and assigned to the present assignee, utilizes fixed pin electrode matrices or similar discharge sources to generate charged particles in response to electric actuation signals and an electric field to attract the generated charge to a dielectric medium to form a charge image. Such systems require complicated actuatingcontrols, and the pin arrangements themselves are susceptible to dirt accumulation, corrosion, parasitic capacitance effects and geometrical variations in physical component structure and arrangement, all of which may contribute to imprecision in the discharge pattern and impaired efficiency.

To eliminate some of the defects arising from atmospheric exposure of multiple discharge sources, other prior art methods have employed a single cathode isolated within a cathode ray tube to generate the necessary electrons. Once generated, the electrons may be directed out of the tube in the form of a beam towards a dielectric medium by a strong electric field, ionizing the surrounding air during transit to produce additional electrons for deposition. In such devices, complex controls and high voltages are required to produce, accelerate and manipulate the electron beam. Furthermore, during traversal fo the substantial tube-to-dielectric distance, divergence of electrons from the desired beam path and scattering caused by particle collisions result in a loss of precision in the resultant charge image.

Thus, in the prior art, adverse effects upon electrostatic printer performance have resulted from constant subjection of permanently placed charge sources to corrosive atmospheric elements and from scattering of charged particles in transit by atmospheric particles. At the same time, geometrical constraints on performance have manifested themselves in inadequate tolerances in size and spacing between co-operating discharge devices and in divergence-conducive spacing between charge source and printing medium. To these undesirable factors must finally be added the disadvantages of prior art control and actuation mechanisms.

SUMMARY OF THE INVENTION Accordingly, the invention contemplates the solution of these prior art problems by providing a single charge source whose presence and hence exposure in the atmosphere surrounding the medium is transitory, whose location from the medium is precisely controllable to within a minimal distance and whose physical basis for operation eliminates cathode-electrode particle beam generation and its associated disadvantages.

It is, therefore, an object of the invention to improve performance and simplify the apparatus of an electrostatic printer,

It is a concurrent object of this invention to provide a simplified and improved method and apparatus for accomplishing charge deposition in an electrostatic printer.

A further object of this invention is to provide charged particles for use in an electrostatic deposition process solely by ionization of air or other gases near a printing medium.

An additional object of this invention is' to produce a controlled, transitory point source of ionsat a minimal distance from a printing medium.

It is yet another object of this invention to provide an electrostatic printing device employing a spatially modulated and focused laser to produce a source of ionized particles from air molecules.

These and other objects and advantages are accomplished by focusing coherent electromagnetic radiation to a point whereat gaseous molecules are ionized and then directly attracted to'a printing medium. Electrooptical manipulation and modulation of this point charge source by known methods facilitates the selective, multi-dimensional charge deposition necessary to form a charge image. This charge image may then be developed by known techniques to form a final print.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects and advantages of the invention, together with other advantages attainable by its use, will be apparent from the following detailed description of the invention read in conjunction with'the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION As shown in FIG. 1, the electrostatic printer of the preferred embodiment of the invention includes a laser source 11, a light modulator 12, a light deflection system 13, a control signal source 14, and a print head station 15. The print head station 15 shown in FIG. 2 includes two parallel plate electrodes 16, 17, encompassing an air gap 18 and a print medium 19 and connected to a potential source 20. The electrode 16 is transparent to laser light, preferably comprising a glass layer 21 and a transparent, conductive tin oxide layer 22. The print medium 19, a dielectric coated paper, is in sliding contact with the surface of the electrode 17.

In operation, the laser source 11 generates a laser beam which is modulated by on-off signals delivered to the modulator 12 from a control signal source 14. The modulated beam then passes to a light deflection system 13 which focuses the beam and directs it to the desired location in accordance with signals from the control signal source 14. Such a deflection system may provide two coordinate X-Y" scanning or single line scanning. The construction and operation of the abovementioned optical components and system are wellknown in the art as illustrated by US. Pat. No. 3,154,371 issued to W. R. Johnson on Oct. 27, 1964.

The focus of the laser beam 23 is adjusted so that a focal point 24 forms just adjacent the printing medium 19. As the laser power is increased, the energy concentrated at the focal point 24 increases toward a threshold where complete air break down and sparking occurs. Below this spark threshold, the focused energy will produce the desired ionization of air particles, which then serve as the charged particles in the charge deposition process.

In the preferred embodiment, ions and NJ ions are produced at the focal point, the 0 ion being predominant in number. The electric field 25 created between the electrodes 16, 17, thus forces the 0; ions to the dielectric coating above the electrode 17 and the NJ ions toward the electrode 16. Therefore, as the laser energy focal point 14 selectively scans the dielectric 19, a pattern of 0; charge spots of very minute size, l microns, are deposited on the media, forming a charge image.

Any suitable method may be used to present the dielectric coated media to the laser ion source and electric field and develop the resultant charge image. One such method is that disclosed in US. Pat. No. 2,995,894 issued to H. Epstein on Oct. 11, 1960, and assigned to the present assignee.

In an alternate embodiment of the invention shown in FIGS. 3 and 4 the light modulator 12 of FIG. 1 may be eliminated infavor of the controlled voltage source 26 of FIG. 4. The purpose of such a change is to enable reversing the polarity of the electric field 27 between the electrodes 28 and 29. As a consequence of a polarity reversal, 0 ions which were once attracted toward the dielectric 30 will now be impelled toward the electrode 28. Thus, when the focal point 31 is adjacent an area of the dielectric 30 to be left vacant, a control signal from the control signal source 31 of FIG. 3 to the voltage source 26 reverses the polarity of the electric field, preventing generated 0 charge from accumulating on the dielectric surface 30.

By similarly controlling the voltage source 26 of FIG. 4 in a system configured in accordance with FIGS. 3 and 4, the further advantages of a two color print may be obtained. If both positive and negative ions are present at the focal point 31, controlling the polarity of the electric field permits the selective deposition of either species of charge. With the electric field oriented as shown in FIG. 4, an area subjected to the laser focal point will be charged with negative ions. If it is then desired to place a charge of opposite polarity on an adjacent area, the laser focal point is deflected to that area, and a control signal is applied to source 26 to cause a reversal in the electric field polar ity so that the positive charged particles developed above that area are compelled to the surface 30. A developing process which produces a different color for each of the respective charge varieties may then be used to produce a multicolor final print.

Obviously, many other variations and modifications of the present invention are possible in light of the teachings above, and it is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An electrostatic printer for recording a charge image on the first of two surfaces of a printing medium in response to signals representing said image in terms of a plurality of positions on said first surface and the desired presence or absence of charge at each of said positions comprising:

a printing station including an electromagnetic radiation transmissive electrode disposed opposite said first surface and a second electrode disposed adjacent said second surface; a source of coherent electromagnetic radiation disposed outside of said printing station;

means for modulating said radiation in accordance with said signals;

means for focusing and selectively deflecting said radiation in response to said signals through said radiation transmissive electrode to selectively form a point source of charge above said first surface; and

means for applying a voltage across said electrodes for attracting said charge to said first surface to form said image.

2. An electrostatic printer for recording a charge image on the first of two surfaces of a printing medium in response to signals representing said image in terms of a plurality of positions on said first surface and the desired presence or absence of charge at each of said positions comprising:

a printing station including an electromagnetic radiation transmissive electrode disposed opposite said first surface and a second electrode disposed adjacent said second surface;

means for focusing and selectively deflecting said radiation in response to said signals through said radiation transmissive electrode to selectively form a point source of charge above said first surface;

means for applying a voltage across said electrodes for attracting said charge to said first surface to form said image; and

means for controlling the polarity of said voltage in response to said signals. 

1. An electrostatic printer for recording a charge image on the first of two surfaces of a printing medium in response to signals representing said image in terms of a plurality of positions on said first surface and the desired presence or absence of charge at eaCh of said positions comprising: a printing station including an electromagnetic radiation transmissive electrode disposed opposite said first surface and a second electrode disposed adjacent said second surface; a source of coherent electromagnetic radiation disposed outside of said printing station; means for modulating said radiation in accordance with said signals; means for focusing and selectively deflecting said radiation in response to said signals through said radiation transmissive electrode to selectively form a point source of charge above said first surface; and means for applying a voltage across said electrodes for attracting said charge to said first surface to form said image.
 2. An electrostatic printer for recording a charge image on the first of two surfaces of a printing medium in response to signals representing said image in terms of a plurality of positions on said first surface and the desired presence or absence of charge at each of said positions comprising: a printing station including an electromagnetic radiation transmissive electrode disposed opposite said first surface and a second electrode disposed adjacent said second surface; means for focusing and selectively deflecting said radiation in response to said signals through said radiation transmissive electrode to selectively form a point source of charge above said first surface; means for applying a voltage across said electrodes for attracting said charge to said first surface to form said image; and means for controlling the polarity of said voltage in response to said signals. 